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The Imperius Curse forces the victim to obey the caster's commands. Along with the Cruciatus Curse and the Killing Curse, the Imperius Curse is considered to be one of the most terrible curses in the magical world, called the Unforgivable Curses; the use of any of the three on another human being is punishable by a life sentence in Azkaban.

Beginner warning: Details follow which you may not wish to read at your current level.

Unlike the Killing Curse and the Cruciatus Curse, it is possible to fight this curse. However, only very skilled wizards are able to fight it, and only very powerful ones are able to cast it – it takes significant mental strength to impose your will upon another. Interestingly, Harry Potter seems to have an innate ability to fight the curse, while Ron Weasley is more susceptible to it than average.

When called upon to name the three Unforgivable Curses, it is Ron who comes up with this one. Professor Moody comments that this curse had made things difficult for the Ministry, as one could never be sure whether the person you had arrested was a true Death Eater, or was doing the will of a Death Eater who had remained hidden.

We see a number of people controlled by means of the Imperius curse throughout the series in and after the fourth book. Harry himself has need to cast this curse in Harry Potter and the Deathly Hallows. The description that we receive at the time, of the action of the curse on the caster, lead us to believe that while a wand is necessary to cast this curse, the actions are somewhat longer-lasting; Harry, for instance, casts this spell on the goblin Bogrod, then on the Death Eater Travers, and has them both under his control at the same time. One gets the feeling that it is only necessary to actually have the wand actively engaged with the subject of the curse while he is getting his instructions; as is seen at the end of that episode, Travers has been instructed to hide himself, and he continues to serenely do so even after Harry has gone.

The effect of the curse on both the subject and the caster are described in the book. The subject of the spell apparently acts according to the caster's wishes because the spell provides a great serenity coupled with a clear set of instructions on what needs to be done. Basically this is an imposition of the mind-set of the slave, who has no responsibility save that of carrying out the instruction he has been given. It is because this mind-set is so comfortable that it can be hard to free slaves in any meaningful way.

Intermediate warning: Details follow which you may not wish to read at your current level.

The list of people we know who are controlled this way is quite large.

In Harry Potter and the Goblet of Fire, we learn that initially Barty Crouch is being controlled by his father, Bartemius. Then, Bartemius is being controlled by Wormtail, while Alastor Moody is being controlled by Barty. There is also a stretch where, possibly, Bertha Jorkins is being controlled by either Wormtail or Voldemort. Voldemort tries to control Harry with this curse, and fails. It is perhaps ironic that Harry's ability to throw off the curse was nurtured by Barty Crouch, who claims to be Voldemort's greatest supporter.

As has been said, Harry, and possibly others, have learnt how to fight the Imperius curse from Barty Crouch, impersonating Mad-Eye Moody. While this teaching does fit Moody's character, and thus aids Barty's disguise, it is possible that Barty Crouch, who had had to suffer this curse for multiple years and finally managed to overcome it, may have had a more personal interest in teaching Defence against this specific Dark Art.

We see relatively few instances of the curse in Harry Potter and the Half-Blood Prince. The story is littered with techniques meant to detect people under the Imperius curse or disguised by other means, which seem marginally effective; in fact, when we first see Tonks on the Hogwarts Express, we are almost led to believe that she is under the Imperius curse. There is an interesting case of Imperius-by-proxy as well; Draco Malfoy admits at the end of the book that he has placed Madam Rosmerta under the Imperius curse, and it seems that while under control, Madam Rosmerta put Katie Bell under the Imperius curse as well.

In Harry Potter and the Deathly Hallows, we see two people under the Imperius curse in the first five chapters: Pius Thicknesse, and Stan Shunpike. Once the Ministry falls, we can safely assume there will be many others, but they don't directly affect the story, except for the goblin Bogrod and the Death Eater Travers, as mentioned above.